Polyurethane flexible foams are well known commercial products with a myriad of uses. The majority of such polyurethane flexible foams may be roughly divided into three art-recognized divisions: free rise slab foam; hot molded foam; and cold molded foam. In addition, each of these broad categories may be further subdivided into one-shot foams and prepolymer foams.
Free rise slab foam constitutes the largest production of polyurethane foam. In free rise slab foam, the reactive ingredients flow from a mixhead onto a moving conveyor where the foam is allowed to rise freely. In high resilience slab foam, the formulation typically contains one or more di- or polyisocyanates, generally toluene diisocyanates (TDI); a polymer polyol containing a dispersed polymer particle phase in a continuous polyol phase, the latter being an oxyethylene-capped polyoxypropylene polyether polyol containing in excess of 50 mol percent primary hydroxyl groups; a low molecular weight chain extender/cross-linker, generally diethanolamine; a polyether silicone foam-stabilizing surfactant; amine and tin catalysts; and an amount of water as a reactive blowing agent effective to provide the desired target foam density.
Slab foam is predominantly open celled, and once prepared, is generally sliced to suitable thickness for the desired end use: carpet underlay, furniture seat cushions, weather stripping, packaging materials, and the like. However, foam products derived from slabstock are generally limited to planar products. Thus, slabstock foam is unsuitable for applications such as contoured seating cushions, particularly automotive seating, arm rests, head rests, vehicle dashboards, and the like. For these applications, molded polyurethane foam must be utilized.
Hot molded polyurethane foam is prepared by injecting the polyurethane reactive formulation into a vented mold which is then placed into a curing oven. The mold is generally heated prior to introduction of the foam formulation, is cured at considerably elevated temperature, and remains at an elevated temperature during demolding, which creates a safety problem due to the possibility of human contact with the hot mold. The time required to cure the polyurethane foam product requires that numerous identical and expensive molds be used for high volume production. As the foam is slow curing, a considerable amount of foam escapes from the mold vents during foaming, and thus represents a considerable waste of raw materials. The hot molding process is quite energy intensive.
Due to the deficiencies of the hot molded foam process, cold molded polyurethane foam has largely supplanted hot molded foam in the United States, although hot molding is still widely practiced in Europe and the Pacific Rim. In cold molding, the reactive polyurethane formulation is introduced into a mold, generally at modestly elevated temperature, for example 150.degree. F. (66.degree. C.), and demolded after a curing cycle of only about 2 to 5 minutes, without in-mold oven cure. Following demold, cell opening is accomplished by crushing, or the cell opening may be done in the mold through use of timed pressure release (TPR) or timed partial pressured release (TPPR), or combinations of these techniques with modest mechanical crushing. The foam is generally then cured at elevated temperature. The cold molding process enables rapid production with a limited number of molds; largely obviates the potential of burns due to the lower mold temperature; and uses substantially less raw materials due to a much smaller amount of foam exuding from the mold vents, which are also typically much smaller than those employed in hot molding.
However, the benefits of cold molding are partially offset by increases in the cost of the foam formulation ingredients. The isocyanate and polyol components must react in a short time to provide high productivity in cold molding. One method of providing short reaction times in the cold molding process is to employ isocyanate terminated prepolymers in which a substantial portion of total polyol is essentially "prereacted". Moreover, prepolymer systems offer the foam producer a "pre-tested" formulation generally involving fewer reactive components, thus lowering the scrap rate, which may be offset by higher raw material costs in some instances. Many foam producers continue to specify one-shot systems, however.
In one-shot systems, in order to achieve the necessary reactivity of the isocyanate and polyol components, it has been necessary to employ polyoxyalkylene polyols having a high primary hydroxyl content, for example greater than 70 mol percent and generally greater than 80 mol percent, whether used as such, or as the continuous phase of a polymer polyol component. The high primary hydroxyl content is necessary to provide rapid cure through reaction with the isocyanate component. Use of low primary hydroxyl polyols leads to foam collapse in cold molded foams. When the polyether polyol of the polyol component or the base polyol of a polymer polyol portion of the polyol component are prepared by base catalyzed oxyalkylation, providing a high primary hydroxyl content merely involves capping a predominantly polyoxypropylene polyol with oxyethylene moieties by introduction of ethylene oxide alone during the last stage of the oxyalkylation. By employing approximately 15-25 weight percent ethylene oxide based on the total polyol weight in the capping phase of the oxyalkylation, primary hydroxyl contents of from 70 mol percent to in excess of 90 mol percent may be obtained. However, the high proportion of relatively hydrophilic terminal polyoxyethylene moieties can adversely affect foam properties, particularly humid aged compression set and wet set.
In the 1960's, double metal cyanide complex catalysts were introduced for oxypropylation. One of the benefits of such catalysts is the relatively low levels of unsaturation introduced into the polyol product through the rearrangement of propylene oxide into allyl alcohol and the subsequent oxypropylation of this monofunctional species. Measurement of unsaturation is generally considered reflective of the monol content of the polyoxyalkylene polyol product. With double metal cyanide complex catalysis, unsaturations in the range of 0.017 to 0.020 meq/g were achieved, as compared to normal, base-catalyzed unsaturations in the range of 0.035 to 0.12 meq/g, measured in accordance with ASTM D-2849-69 "Testing Urethane Foam Polyol Raw Materials." The latter value reflects a concentration of monol of from 40 mol percent to in excess of 50 mol percent, as a result of which the measured, actual functionality of the polyol is considerably lower than the theoretical, or "nominal" functionality which is the same as the initiator functionality. However, despite the advantages of double metal cyanide complex catalysts in lowering unsaturation and monol content, the cost/activity ratio of such catalysts coupled with the necessity to purify the polyol product to eliminate the difficulty removable catalyst residues prevented successful commercialization. Improvement in catalyst A activity, as illustrated by U.S. Pat. No. 5,158,992, heightened prospects of commercialization, as well as providing yet lower unsaturation, in the range of 0.015 to 0.018 meq/g. However, removal of catalyst residues still posed a problem.
Most recently, however, the ARCO Chemical Company has developed new double metal cyanide complex catalysts with much higher catalytic activity and unprecedented low levels of unsaturation, the latter in the range of 0.003 to 0.007 meq/g. The use of lower levels of catalysts coupled with higher polyoxyalkylation rates and the use of simple filtration to remove catalyst residues has resulted in commercialization of double metal cyanide complex-catalyzed polyoxypropylene and random polyoxypropylene/polyoxyethylene copolymer polyols.
However, in order to prepare double metal cyanide complex-catalyzed polyols having high primary hydroxyl content, it has thus far been found necessary to prepare the high primary hydroxyl polyoxyethylene cap in the presence of traditional basic catalysts such as potassium hydroxide. Following initial preparation of an all propylene oxide-derived polyol or a random propylene oxide/ethylene oxide-derived polyol by double metal cyanide complex catalysis, basic catalyst is added which destroys, deactivates, or denatures the double metal cyanide catalyst and provides the requisite basic catalyst level necessary for oxyethylene group capping. The polyol product must then be treated to remove basic catalyst residues in the conventional manner.
The process just described is wasteful of double metal cyanide complex, which, if not denatured, can sometimes be reused, and further requires conventional polyol finishing steps to remove basic catalyst, adding to processing time and expense. If ethylene oxide addition is attempted with double metal cyanide complex catalysts without addition of basic catalysts, however, a complex mixture believed to contain considerable quantities of homopolyoxyethylene polymers rather than an oxyethylene capped product are obtained. Thus, the benefits of using double metal cyanide complex-catalyzed polyols in formulations requiring high primary hydroxyl content polyols have not been realized.